KR20180136032A - Redox flow battery system and method for balancing state of charge thereof - Google Patents

Abstract

The present invention relates to a redox flow battery system and to a method for balancing the degree of charging thereof, and more specifically, the redox flow battery system comprises: a first redox flow cell; a secondary redox flow cell; a DC/AC conversion unit electrically connecting the first redox flow cell and the second redox flow cell to each other; and a controller controlling the first and second redox flow cells and the DC/AC conversion unit. Each of the first and second redox flow cells includes a first storage tank storing a first electrolyte and a second storage tank storing a second electrolyte, wherein the DC/AC conversion unit converts the electrical connection between the first and second redox flow cells into direct current to alternating current or alternating current to direct current.

Description

[0001] The present invention relates to a redox flow battery system and a method of balancing its charge balance.

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a redox flow battery system and a charging balancing method thereof, and more particularly, to a redox flow battery system and its charging balancing method capable of minimizing a charging difference between redox flow cells .

In recent years, with the seriousness of power shortage, introduction of natural energy such as wind power generation and solar power generation and stabilization of power system have become a global issue. As one of technologies for this, a large-capacity battery technology for stable output and storage of surplus power is attracting attention.

One of the large capacity batteries is the redox flow battery. A redox flow cell is a device that converts the chemical energy of an electrolyte solution into electric energy through a cell. The redox flow battery has advantages such as large capacity, long life, and accurate monitoring of the state of charge of the battery, which can be regarded as an optimal storage for stabilizing the power system.

On the other hand, the imbalance in the degree of charge between the redox flow cells causes a reduction in the capacity of the system comprising a plurality of redox flow cells and deterioration in performance.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a redox flow battery system capable of improving capacity and performance.

Another object of the present invention is to provide a charging balancing method of a redox flow battery system capable of improving capacity and performance.

According to the concept of the present invention, a redox flow cell system comprises: a first redox flow cell; A secondary redox flow cell; A DC / AC conversion unit electrically connecting the first redox flow cell and the second redox flow cell to each other; And a controller for controlling the first and second redox flow cells and the DC / AC converter. Wherein each of the first and second redox flow cells includes a first storage tank in which a first electrolyte is stored and a second storage tank in which a second electrolyte is stored and wherein the DC / The electrical connection between the battery cells can be converted from AC to AC or from AC to DC.

Wherein the first storage tank of the first redox flow cell is distinguished from the first storage tank of the second redox flow cell and the second storage tank of the first redox flow cell is separated from the second redox flow cell, Can be distinguished from the second storage tank of the flow cell.

The controller may further include: a charge level measuring unit for measuring a charge level of each of the first and second redox flow cells; An information processing unit for calculating a charge difference between the first and second redox flow cells based on the measured charge level; And a control unit for controlling electrical connection between the first and second redox flow cells by controlling the direct current / alternating current switching unit based on a difference in charge between the first and second redox flow cells. have.

The controller may control the pump of each of the first and second redox flow cells to circulate the first electrolyte and the second electrolyte.

In the idle unit before the charging or discharging of the first and second redox flow cells is performed, the DC / AC conversion unit converts the electrical connection between the first and second redox flow cells from the DC to the AC .

According to another aspect of the present invention, a method for balancing the charge of a redox flow battery system includes the steps of measuring a charge level of a first redox flow battery and a second redox flow battery; Calculating a charge difference between the first and second redox flow cells; Converting the electrical connections between the first and second redox flow cells from serial to parallel based on the charge difference; Circulating an electrolyte of each of the first and second redox flow cells for a predetermined period of time to reduce the difference in charge between the first and second redox flow cells; Converting the electrical connections between the first and second redox flow cells into parallel to serial; And charging or discharging the first and second redox flow cells.

Wherein the step of converting the electrical connection between the first and second redox flow cells from serial to parallel is performed when the charge difference is greater than a first reference value and the first reference value is selected within 5% .

Wherein the step of converting the electrical connection between the first and second redox flow cells into parallel is performed when the charge difference is less than the second reference value and the second reference value is selected within 1% .

Wherein the step of reducing the charge difference is performed when the first and second redox flow cells are electrically disconnected from an external power source or an external load, This can be done when the doff-flow cells are electrically connected to external power or external loads.

The first and second redox flow cells may not share an electrolyte solution with each other.

The redox flow battery system according to the present invention can be easily minimized in charge difference between redox flow cells by a simple structure and method. In this way, the capacity and performance of the redox flow battery system can be improved.

1 is a schematic view illustrating a redox flow battery system according to an embodiment of the present invention.
2 is a schematic view illustrating a redox flow cell according to an embodiment of the present invention.
3 is a schematic view illustrating a redox flow cell according to another embodiment of the present invention.
4 is a schematic view for explaining a redox flow cell according to another embodiment of the present invention.
FIG. 5 is a flow chart illustrating a method for balancing the charge level of a redox flow battery system in accordance with embodiments of the present invention.
FIG. 6 is a schematic diagram showing that the electrical connections between redox flow cells of FIG. 1 are converted in parallel. FIG.
7 is a schematic view illustrating a redox flow battery system according to another embodiment of the present invention.
8 is a graph illustrating changes in voltage difference with time for the first and second redox flow cells connected in parallel.
FIG. 9 is a graph comparing experimental results with modeling results of voltage differences between the first and second redox flow cells over time.
FIG. 10 is a graph showing a result of modeling a reduction in charge difference with time between redox flow battery modules. FIG.

In order to fully understand the structure and effects of the present invention, preferred embodiments of the present invention will be described with reference to the accompanying drawings. However, the present invention is not limited to the embodiments described below, but may be embodied in various forms and various modifications may be made. It will be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the spirit or essential characteristics thereof.

In this specification, when an element is referred to as being on another element, it may be directly formed on another element, or a third element may be interposed therebetween. Further, in the drawings, the thickness of the components is exaggerated for an effective description of the technical content. The same reference numerals denote the same elements throughout the specification.

Although the terms first, second, third, etc. in the various embodiments of the present disclosure are used to describe various components, these components should not be limited by these terms. These terms have only been used to distinguish one component from another. The embodiments described and exemplified herein also include their complementary embodiments.

The terminology used herein is for the purpose of illustrating embodiments and is not intended to be limiting of the present invention. In the present specification, the singular form includes plural forms unless otherwise specified in the specification. The terms "comprises" and / or "comprising" used in the specification do not exclude the presence or addition of one or more other elements.

1 is a schematic view illustrating a redox flow battery system according to an embodiment of the present invention.

Referring to FIG. 1, a redox flow battery system 1 according to an embodiment of the present invention includes a first redox flow battery RFB1, a second redox flow battery RFB2, a DC / AC conversion unit CVT ), A power conversion unit (PCS), and a control device 10. Each of the first and second redox flow cells RFB1 and RFB2 may include a positive electrode PE and a negative electrode NE.

The DC / AC conversion unit CVT may convert the electrical connection between the first and second redox flow cells RFB1 and RFB2 from DC to AC or from AC to DC. The DC / AC conversion unit (CVT) may block the electrical connection between the external power source / load and the first and second redox flow cells (RFB1, RFB2). Any device capable of DC / AC conversion between the first and second redox flow cells RFB1 and RFB2 may be used as the DC / AC conversion unit CVT. For example, as shown in FIG. 1, A device using a switch can be used.

For example, in FIG. 1, it can be seen that the electrical connection between the first and second redox flow cells RFB1 and RFB2 is connected in series via the DC / AC conversion unit CVT. 6, the switches of the DC / AC conversion unit CVT are switched to electrically connect the first and second redox flow cells RFB1 and RFB2 in parallel via the DC / AC conversion unit CVT Can be connected.

The power conversion unit (PCS) can be connected to an external power source / load. In other words, the first and second redox flow cells RFB1 and RFB2 can be connected to the external power source / load through the power conversion unit PCS. The power conversion unit PCS may convert power, and the power conversion unit PCS may include a DC / AC converter, for example. Charging or discharging may be performed on the first and second redox flow cells RFB1 and RFB2 connected to the external power source / load through the power conversion unit PCS.

The control device 10 may include a charge level measurement unit 12, a control unit 14, and an information processing unit 16. The charge level measuring unit 12 may measure a state of charge (SOC) of each of the first and second redox flow cells RFB1 and RFB2. The control unit 14 can control the operation of the first and second redox flow cells RFB1 and RFB2. The controller 14 may control the electrical connection (DC or AC) between the first and second redox flow cells RFB1 and RFB2 by controlling the DC / AC conversion unit CVT. The information processing unit 16 may calculate the difference between the measured charge degrees and compare the difference with the predetermined reference value.

Hereinafter, specific examples of the first and second redox flow cells RFB1 and RFB2 will be described with reference to FIGS. 2 to 4. FIG.

2 is a schematic view illustrating a redox flow cell according to an embodiment of the present invention.

Referring to FIG. 2, the redox flow battery RFB according to an embodiment of the present invention includes a cell CEL, a first storage tank 110a, a first pump 120a, a second storage tank 110b, And a second pump 120b.

The first storage tank 110a may store the first electrolyte, and the second storage tank 110b may store the second electrolyte. For example, the first electrolyte may be a cathode electrolyte, and the second electrolyte may be a cathode electrolyte. The first electrolyte may include V ⁴⁺ ions and V ⁵⁺ ions as a vanadium active material, and the second electrolyte may include V ²⁺ ions and V ³⁺ ions as vanadium active materials. Each of the first electrolytic solution and the second electrolytic solution may further include sulfuric acid, hydrochloric acid, or a mixed acid thereof. Sulfuric acid or hydrochloric acid may dissolve the vanadium active material.

The cell CEL includes a positive electrode 102a, a negative electrode 102b, a first electrode 106a in the positive electrode cell 102a, a second electrode 106b in the negative electrode cell 102b, And an ion exchange membrane 104 interposed between the anode and cathode cells 102b. For example, the first electrode 106a may correspond to the anode PE described above with reference to FIG. 1, and the second electrode 106b may correspond to the cathode NE previously described with reference to FIG.

The first electrode 106a and the second electrode 106b may provide an active site for the oxidation / reduction reaction in the anode cell 102a and the cathode cell 102b, respectively. For example, the first electrode 106a and the second electrode 106b may comprise a nonwoven fabric, carbon fiber, or carbon paper.

The first electrolyte in the first storage tank 110a may be introduced into the anode cell 102a through the first pump 120a. In the anode cell 102a, movement of electrons through the first electrode 106a may occur due to charging or discharging. For example, at the time of charging, V ⁵⁺ ions are generated from V ⁴⁺ ions, and electrons are transferred to the first electrode 106a (oxidation). At the time of discharging, V ⁴⁺ ions are generated from V ⁵⁺ ions, and electrons are transferred from the first electrode 106a (reduction). After the oxidation / reduction reaction in the anode cell 102a, the first electrolyte may flow into the first storage tank 110a again. The electrolyte can be circulated between the first storage tank 110a and the anode cell 102a through the first pump 120a.

The second electrolyte in the second storage tank 110b may be introduced into the cathode cell 102b through the second pump 120b. In the cathode cell 102b, electrons may move through the second electrode 106b in response to charging or discharging. For example, during charging, V ²⁺ ions are generated from V ³⁺ ions and electrons are transferred from the second electrode 106b (reduction). At the time of discharging, V ³⁺ ions are generated from V ²⁺ ions and electrons are transferred to the second electrode 106b (oxidation). After completion of the oxidation / reduction reaction in the cathode cell 102b, the second electrolyte may be introduced into the second storage tank 110b again. The electrolyte can be circulated between the second storage tank 110b and the cathode cell 102b through the second pump 120b.

The anode cell 102a and the cathode cell 102b can be separated from each other by the ion exchange membrane 104. [ On the other hand, ion movement, that is, crossover, may occur between the anode cell 102a and the cathode cell 102b through the ion exchange membrane 104. For example, a hydrogen ion (H ⁺ ) may pass through the ion exchange membrane 104 and move between the anode cell 102a and the cathode cell 102b. In addition, some of the vanadium active material ions (V2 ⁺ , V3 ⁺ , V4 ⁺ , ^{V5 +} ) may pass through the ion exchange membrane 104 as well.

When the redox flow cell RFB according to an embodiment of the present invention is completely charged (SOC = 100%), the vanadium active material of the first electrolyte may exist only in the V ^{5 +} ion, and the vanadium active material of the second electrolyte V ²⁺ ions alone. When the redox flow cell RFB according to an embodiment of the present invention is completely discharged (SOC = 0%), the vanadium active material of the first electrolyte may exist only in the V ^{4 +} ion, and the vanadium active material of the second electrolyte V ³⁺ ions alone. When the redox flow cell RFB according to an embodiment of the present invention is charged to half the charge (SOC = 50%), the vanadium active material of the first electrolyte has a ratio of ^{V5 +} ions and V4 ⁺ ions of 1: And the vanadium active material of the second electrolyte may exist in a ratio of V ^{2 +} ion to V ^{3 +} ion in a ratio of 1: 1. In other words, the degree of filling (SOC) of the redox flow cell RFB depends on the ratio of V ^{5 +} ions to V ^{4 +} ions in the first electrolyte and the ratio of V ^{2 +} ions to V ^{3 +} ions in the second electrolyte Can be defined. Generally, the charge of redox flow cell can be obtained by measuring the open-circuit voltage. Assuming that balancing of the anolyte and catholyte is maintained, the concentration of oxidation / reduction species of anode and cathode electrolytes using the Nernst equation . The ratio of V ⁵⁺ of the total vanadium concentration based on the anolyte solution is the filling degree, and the ratio of V ²⁺ of the total vanadium concentration based on the negative electrode solution is the filling degree.

3 is a schematic view illustrating a redox flow cell according to another embodiment of the present invention.

3, a redox flow battery RFB according to an embodiment of the present invention includes a stack ST, a first storage tank 110a, a first pump 120a, a second storage tank 110b, And a second pump 120b. The stack ST may include a plurality of cells CEL1 to CEL5. For example, the stack ST may include first through fifth cells CEL1 through CEL5. Each of the first through fifth cells CEL1 through CEL5 may be the same as or similar to the cell CEL described above with reference to FIG.

Specifically, each of the first through fifth cells CEL1-CEL5 may include an ion exchange membrane 104. [ Bipolar electrodes 106 may be disposed between the first through fifth cells CEL1 through CEL5. A pair of adjacent cells may share one bipolar electrode 106.

For example, the bipolar electrode 106 between the first cell CEL1 and the second cell CEL2 corresponds to the second electrode 106b previously described with reference to Fig. 2 on the basis of the first cell CEL1 And may correspond to the first electrode 106a described above with reference to FIG. 2 on the basis of the second cell CEL2. Electrons are transferred from the cathode cell 102b of the first cell CEL1 to the bipolar electrode 106 and the electrons transferred to the bipolar electrode 106 pass through the anode of the second cell CEL2 Cell 102a. Electrons are transferred from the positive electrode cell 102a of the second cell CEL2 to the bipolar electrode 106 and the electrons transferred to the bipolar electrode 106 pass through the negative electrode of the first cell CEL1 Cell 102b.

The first collecting electrode 108a and the second collecting electrode 108b may be disposed at both ends of the stack ST. The first collector electrode 108a may be adjacent to the anode cell 102a of the first cell CEL1 and the second collector electrode 108b may be adjacent to the cathode cell 102b of the fifth cell CEL5. have. For example, the first collector electrode 108a may correspond to the anode PE described above with reference to FIG. 1, and the second collector electrode 108b may correspond to the cathode NE described above with reference to FIG. 1 .

The first electrolyte may be introduced into the anode cells 102a of the first through fifth cells CEL1 through CEL5 from the first storage tank 110a through the first pump 120a. The second electrolyte may be introduced into the cathode cells 102b of the first through fifth cells CEL1-CEL5 from the second storage tank 110b through the second pump 120b. The first to fifth cells CEL1 to CEL5 in the stack ST may share the first electrolyte in one first storage tank 110a. The first to fifth cells CEL1 to CEL5 in the stack ST may share the second electrolyte in one second storage tank 110b.

The first to fifth cells CEL1 to CEL5 in the stack ST shown in Fig. 3 are exemplary, and the number of cells in the stack ST and their arrangement can be appropriately selected by those skilled in the art.

4 is a schematic view for explaining a redox flow cell according to another embodiment of the present invention.

Referring to FIG. 4, a redox flow battery RFB according to an embodiment of the present invention includes a module MD, a first storage tank 110a, a first pump 120a, a second storage tank 110b, And a second pump 120b. The module MD may include a plurality of stacks ST1 to ST6. For example, the module MD may include first through sixth stacks ST1-ST6. Each of the first to sixth stacks ST1 to ST6 may be the same as or similar to the stack ST described above with reference to FIG.

The first to sixth stacks ST1 to ST6 may be connected to each other in series and / or in parallel. For example, the first through third stacks ST1, ST2, and ST3 may be connected in series with each other, and the fourth through sixth stacks ST4, ST5, and ST6 may be connected in series with each other. The first through third stacks ST1, ST2, and ST3 and the fourth through sixth stacks ST4, ST5, and ST6 may be connected in parallel. Through the electrical connection between the first to sixth stacks ST1 to ST6, the module MD can have one anode PE and one cathode NE described above with reference to FIG.

The first to sixth stacks ST1 to ST6 in one module MD may share the first electrolyte in one first storage tank 110a. The first to sixth stacks ST1 to ST6 in one module MD may share a second electrolyte in one second storage tank 110b.

The first to sixth stacks ST1 to ST6 and their electrical connections in the module MD shown in Fig. 4 are exemplary and the number of stacks in the module MD and the electrical connection relationship therebetween can be determined by a person skilled in the art It can be selected appropriately.

2 to 4, specific examples of the first and second redox flow cells RFB1 and RFB2 of FIG. 1 have been described. Referring again to FIGS. 1 to 4, the first redox flow battery RFB1 may include one first storage tank 110a and one second storage tank 110b. The second redox flow battery RFB2 may also include one first storage tank 110a and one second storage tank 110b. The first storage tank 110a of the first redox flow battery RFB1 is distinguished from the first storage tank 110a of the second redox flow battery RFB2 and the first storage tank 110b of the first redox- 2 storage tank 110b can be distinguished from the second storage tank 110b of the second redox flow battery RFB2. In other words, the first and second redox flow cells RFB1 and RFB2 can not share one first storage tank 110a with each other. The first and second redox flow cells RFB1 and RFB2 can not share one second storage tank 110b with each other.

A charge difference ΔSOC between the first redox flow battery RFB1 and the second redox flow battery RFB2 can be generated. This is because the first redox flow battery RFB1 and the second redox flow battery RFB2 do not share the first storage tank 110a and the second storage tank 110b with each other. In other words, the ratio of the V ^{5 +} ions and the V ^{4 +} ions of the first electrolyte in the first storage tank 110a of the first redox flow battery RFB1 is set to the first storage of the second redox flow battery RFB2 The ratio of the V ^{5 +} ion to the V ^{4 +} ion of the first electrolyte in the tank 110a may be different. Further, the ratio of the V ^{2 +} ions and the V ^{3 +} ions of the second electrolyte in the second storage tank 110b of the first redox flow battery RFB1 is set to the second storage tank May be different from the ratio of the V ^{2 +} ion and the V ^{3 +} ion of the second electrolyte in the second electrolyte solution 110b.

FIG. 5 is a flow chart illustrating a method for balancing the charge level of a redox flow battery system in accordance with embodiments of the present invention. FIG. 6 is a schematic diagram showing that the electrical connections between redox flow cells of FIG. 1 are converted in parallel. FIG.

1, 5 and 6, the redox flow battery system 1 may go through an idle period (IP) and a charge / discharge period (CD). The idle unit IP may be a period during which the redox flow battery system 1 is storing electrochemical energy without substantial charge or discharge. The charge / discharge unit CD may be a period during which the redox flow battery system 1 is being charged or discharged according to the operation of the external power source / load.

The idle unit IP according to embodiments of the present invention includes a step S110 of measuring the degree of filling of each of the first and second redox flow cells RFB1 and RFB2, (S120) determining whether or not the charging degree difference? SOC is larger or smaller than a predetermined first reference value (S130), calculating a charging degree difference? SOC between the first and second reference values RFB1 and RFB2 The first pump 120a in each of the first and second redox flow cells RFB1 and RFB2 converts the electrical connection between the two redox flow cells RFB1 and RFB2 from serial to parallel at step S140, (S150) circulating the first electrolyte and the second electrolyte for a predetermined time by operating the second pump 120b and the second pump 120b to switch the first and second redox flow cells RFB1 and RFB2 (S160) of determining whether the charge difference difference? SOC is larger or smaller than a predetermined second reference value, and determining whether the first and second redox flow cells (RFB1, RFB2) (S160) of converting the electrical connection between the parallel and serial circuits.

Specifically, the degree of filling of each of the first and second redox flow cells RFB1 and RFB2 may be measured through the charge degree measuring unit 12 of the controller 10 in the idle unit IP (S110 ). Theoretically, the first electrolyte solution and the second electrolyte solution in the first storage tank 110a and the second storage tank 110b of the first redox flow battery RFB1 are analyzed to prepare the first redox flow battery RFB1 1 charge can be measured and the first electrolyte and the second electrolyte in the first storage tank 110a and the second storage tank 110b of the second redox flow cell RFB2 are analyzed to form a second redox flow The second degree of filling of the battery RFB2 can be measured. Generally, as described above, the open circuit voltage of each redox flow cell can be measured to indicate the degree of charge.

Based on the measured first charge and the measured second charge, the charge difference (? SOC), which is the difference between them, can be calculated (S120). Specifically, the information processing unit 16 of the control device 10 can calculate the charge difference ΔSOC.

It may be confirmed (or determined) whether the charge difference ΔSOC is larger or smaller than a predetermined first reference value (S130). The information processing unit 16 of the control device 10 can confirm (or determine) whether the charge degree difference? SOC is larger or smaller than the first reference value. The first reference value may be arbitrarily selected by the user within an appropriate range. Specifically, the first reference value may be selected within the range of 5% to 20%. For example, the first reference value may be set to 10%. In this case, the information processing unit 16 can determine whether the charge difference? SOC is larger or smaller than 10%.

The electrical connection between the first and second redox flow cells RFB1 and RFB2 may be converted from serial to parallel when the charge degree difference? SOC is greater than the first reference value (S140). The switch of the DC / AC conversion unit CVT can be switched through the control unit 14 of the controller 10 as shown in FIG. Thereby, the electrical connection between the first and second redox flow cells RFB1 and RFB2 can be converted from serial to parallel. If the electrical connection between the first and second redox flow cells RFB1 and RFB2 has already been converted in parallel, the step of converting from serial to parallel (S140) may be omitted. The DC / AC conversion unit (CVT) may block the electrical connection between the external power source / load and the first and second redox flow cells (RFB1, RFB2).

The first pump 120a and the second pump 120b of the first redox flow battery RFB1 are operated to circulate the first electrolyte solution and the second electrolyte solution in the first redox flow battery RFB1 . At the same time, the first pump 120a and the second pump 120b of the second redox flow battery RFB2 are operated to cause the first electrolyte solution and the second electrolyte solution to circulate in the second redox flow battery RFB2 (S150). Specifically, the control unit 14 of the control apparatus 10 can operate the first pump 120a and the second pump 120b of the first redox flow battery RFB1, The first pump 120a and the second pump 120b of the RFB2 can be operated.

The first and second redox flow cells RFB1 and RFB2 may not be electrically connected to the external power source / load even if the first and second electrolytic solutions are circulated. Since the first and second redox flow cells RFB1 and RFB2 are connected in parallel with each other, the first and second redox flow cells RFB1 and RFB2 are turned on while the first and second electrolytic solutions are circulating, A current can flow between them. While the first and second electrolytic solutions are circulating, the redox flow cell having a relatively high filling degree is discharged, and the redox flow battery having a relatively low filling degree can be charged by receiving energy from the discharged battery .

For example, when the degree of filling of the first redox flow cell RFB1 is 50% and the degree of filling of the second redox flow cell RFB2 is 30%, the first and second redox flow cells RFB1 , RFB2), the first redox flow cell RFB1 can be discharged and the second redox flow cell RFB2 can be charged. This can be continued until the charge of the first redox flow battery RFB1 and the charge of the second redox flow battery RFB2 become substantially equal. When the first redox flow cell RFB1 and the second redox flow cell RFB2 are of the same capacity, the first redox flow cell RFB1 and the second redox flow cell RFB2 The degree of filling can all be 40%.

The energy stored in the first redox flow battery RFB1 and the second redox flow battery RFB2 may be used in operating the first pump 120a and the second pump 120b. If the first pump 120a and the second pump 120b are operated at a high power, the difference in charge between the first redox flow battery RFB1 and the second redox flow battery RFB2, Can be reduced more quickly, but the energy stored in the first redox flow cell RFB1 and the second redox flow cell RFB2 may be excessively consumed. If the first pump 120a and the second pump 120b are operated at a low rate, the charge difference ΔSOC between the first redox flow battery RFB1 and the second redox flow battery RFB2 is It may be reduced more slowly, resulting in inefficiency. Accordingly, the first pump 120a and the second pump 120b are operated with optimum efficiency by appropriately comparing the rate of decrease of the charge difference (DELTA SOC) with the energy consumption rate of the first pump 120a and the second pump 120b can do.

After the first and second electrolytes are circulated for a certain period of time, the degree of filling of each of the first and second redox flow cells RFB1 and RFB2 can be measured and the charge difference? SOC between them can be calculated again . The information processing unit 16 of the control apparatus 10 can calculate the charge degree difference? SOC.

It may be confirmed (or determined) whether the charge difference ΔSOC is larger or smaller than a predetermined second reference value (S160). The information processing unit 16 of the control apparatus 10 can confirm (or determine) whether the charge difference difference? SOC is larger or smaller than the second reference value. The second reference value may be equal to or smaller than the first reference value. Specifically, the second reference value may be selected from 1% to 10%. For example, the second reference value may be set to 5%. In this case, the information processing unit 16 can determine whether the charge difference? SOC is larger or smaller than 5%.

When the charge degree difference? SOC is larger than the second reference value, the first and second electrolytes in the first and second redox flow cells RFB1 and RFB2 can be circulated again. This process is controlled through the control unit 14 of the controller 10 and can be repeated until the charge difference ΔSOC becomes smaller than the second reference value.

The electrical connection between the first and second redox flow cells RFB1 and RFB2 can be converted from parallel to serial when the charge difference difference ΔSOC is smaller than the second reference value S160. The switch of the DC / AC converting unit CVT can be switched through the control unit 14 of the controller 10 as shown in FIG. Thereby, the electrical connection between the first and second redox flow cells RFB1 and RFB2 can be converted from parallel to serial.

Thereafter, the idle state (IP) state of the redox flow battery system 1 is released and the redox flow battery system 1 can enter the charge / discharge machine CD. The charge / discharge unit CD may include charging or discharging the redox flow battery system 1 (S200). With the operation of the external power source / load, the first and second redox flow cells (RFB1, RFB2) in the redox flow battery system 1 can be charged or discharged.

The charge balancing method of the redox flow battery system according to embodiments of the present invention can maintain the charge difference between the redox flow cells during the idle period IP to be minimized. Compared to the method of mixing the electrolytes of the redox flow cells, the present invention can reduce the stored energy loss and simplify the charging difference between redox flow cells with a simple configuration and method. By minimizing the charge difference, the present invention can utilize substantially all of the energy of the redox flow cells at the time of charging or discharging the redox flow battery system.

7 is a schematic view illustrating a redox flow battery system according to another embodiment of the present invention. In the present embodiment, detailed description of technical features overlapping with those described with reference to Figs. 1 to 6 will be omitted, and differences will be described in detail.

Referring to FIG. 7, a redox flow battery system 1 according to an embodiment of the present invention includes a first redox flow cell RFB1, a second redox flow cell RFB2, a third redox flow cell (RFB3), a DC / AC conversion unit (CVT), a power conversion unit (PCS), and a control device (10). Compared with the redox flow cell system 1 described above with reference to FIG. 1, the redox flow cell system 1 according to embodiments of the present invention can include three or more redox flow cells.

Referring to FIG. 7, the first to third redox flow cells RFB1, RFB2 and RFB3 may be electrically connected to each other through a DC / AC conversion unit CVT. The first to third redox flow cells RFB1, RFB2 and RFB3 may be electrically connected to the power conversion unit PCS through a DC / AC conversion unit CVT. The DC / AC conversion unit CVT may convert the electrical connection between the first to third redox flow cells RFB1, RFB2 and RFB3 from DC to AC or from AC to DC. Any device capable of DC / AC conversion between the first to third redox flow cells (RFB1, RFB2, RFB3) can be used without limitation as the DC / AC conversion unit (CVT).

5 and 7, the first to third charge diagrams of the first to third redox flow cells RFB1, RFB2 and RFB3 are read through the charge degree measurement unit 12 of the controller 10 Respectively (S110). The information processing unit 16 of the control device 10 calculates the charge difference (? SOC) between the first to third redox flow cells (RFB1, RFB2, RFB3) based on the measured first to third charge metrics (S120).

In this embodiment, the charge difference difference? SOC of the first to third redox flow cells RFB1, RFB2 and RFB3 is a difference between the highest charge and the lowest charge among the first to third charge diagrams Lt; / RTI > For example, if the degree of filling of the first redox flow battery RFB1 is 50%, the degree of filling of the second redox flow battery RFB2 is 40%, and the filling degree of the third redox flow battery RFB3 is 30 %, The charge difference (? SOC) may be 20%.

The electrical connection between the first to third redox flow cells RFB1, RFB2 and RFB3 may be converted from serial to parallel when the charge difference? SOC is larger than the first reference value (S140). The first pump 120a and the second pump 120b of each of the first to third redox flow cells RFB1, RFB2 and RFB3 are operated to drive the first to third redox flow cells RFB1 and RFB2 , RFB3) can be performed (S150). Thereby, the degrees of filling of the first to third redox flow cells (RFB1, RFB2, RFB3) can be substantially the same or similar to each other.

Experimental Example 1

A test facility for a redox flow cell containing one cell was prepared. The first redox flow battery (VRB1) and the second redox flow battery (VRB2) were connected in parallel with each other. The test conditions are as follows.

20cm2 (cell area), 1.7M electrolyte, FAP450 membrane (ion exchange membrane), SGL felt (electrode)

Initial SOC difference between first and second redox flow cells: 13.5% (Ah 0.309 Ah difference, theoretical capacity 2.278 Ah)

The change in the voltage difference with time is measured for the first and second redox flow cells (VRB1 and VRB2) connected in parallel, and is shown in FIG.

Referring to FIG. 8, after 2 to 3 hours have elapsed from the parallel connection, the first redox flow battery (VRB1) having a low initial charge level is charged by the second redox flow battery (VRB2) Can be confirmed. This process is performed spontaneously and no additional energy loss occurs.

The reduction of the voltage difference with time between the first and second redox flow cells (VRB1, VRB2) depends on the initial voltage of each cell, the capacity and the internal resistance of the parallel-connected conductors. Based on this, the modeling results of the voltage difference between the first and second redox flow cells (VRB1, VRB2) over time are compared with the experimental results and shown in FIG.

Referring to FIG. 9, it can be confirmed that the modeling result and the experimental result agree very well.

Experimental Example 2

9, it is confirmed that the experimental value and the modeling result are in good agreement. Thus, for two redox flow battery modules, when their charge difference (ΔSOC) is 20%, the decrease in charge difference (ΔSOC) over time when they are connected in parallel is modeled, Respectively. Each of the redox flow battery modules used in the modeling was a 250 kW / 1 MWh high capacity module in which 40 stacks were connected in series and in parallel with each other. Specifically, each stack is made up of 50 cells, and eight stacks out of 40 stacks are connected in series to form one set, and these sets are connected in parallel to each other (8X5).

Referring to FIG. 10, it took 2.2 hours for the charge difference (? SOC) to decrease from 20% to 10%. It took 2.4 hours to reduce the charge difference (ΔSOC) from 10% to 5%. It took 5.4 hours for the charge difference (ΔSOC) to decrease from 5% to 1%.

It can be seen that the larger the charge difference difference (ΔSOC), the greater the decrease in the charge difference ΔSOC (ie, the slope of the graph) with respect to the unit time. Therefore, in the balancing method according to the embodiments of the present invention, when the reference value of the charge difference (? SOC) is set relatively low (for example, when the second reference value is set to 1% or less in FIG. 5) The redox flow cells must be driven for a very long time. This increases the energy consumption for driving the pump and leads to battery inefficiency. Therefore, it is possible to efficiently balance the redox flow battery system by maximizing the reduction amount of the charge difference ([Delta] SOC) by selecting the reference value of the charge difference difference [Delta] SOC as an appropriate reference value.

Referring to the results of FIG. 10, it was confirmed that the larger the charge difference (ΔSOC) between the redox flow cells, the greater the decrease in the charge difference (ΔSOC) according to the unit time. In other words, the greater the charge difference (? SOC) between redox flow cells, the greater the balancing efficiency of the balancing method according to the present invention. The balancing method of the present invention basically utilizes the difference in open-circuit voltage between each redox flow battery. Although the charge difference (ΔSOC) between the redox flow cells is generally the same, the difference in the open voltage can be increased as the chargeability (SOC) of the redox flow battery is further away from 50%. This maximizes the balancing efficiency immediately after a full charge and a complete discharge. Accordingly, even when the idle unit is used, balancing efficiency can be maximized when the balancing method according to the present invention is applied when the SOC of each redox flow battery is close to 0% or 100%. In addition, the above-described charge difference may be applied based on the open-circuit voltage difference suitable for each redox flow cell.

While the present invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, It will be understood. It is therefore to be understood that the above-described embodiments are illustrative and not restrictive in every respect.

Claims (10)

A first redox flow cell;
A secondary redox flow cell;
A DC / AC conversion unit electrically connecting the first redox flow cell and the second redox flow cell to each other; And
And a controller for controlling the first and second redox flow cells and the DC / AC converter,
Wherein each of the first and second redox flow cells includes a first storage tank in which a first electrolyte is stored and a second storage tank in which a second electrolyte is stored,
And the DC / AC conversion unit converts the electrical connection between the first and second redox flow cells into a direct current to an alternating current or an alternating current to a direct current.
The method according to claim 1,
Wherein the first storage tank of the first redox flow cell is distinguished from the first storage tank of the second redox flow cell,
Wherein the second storage tank of the first redox flow cell is distinct from the second storage tank of the second redox flow cell.
The method according to claim 1,
The control device comprising:
A charge level measuring unit for measuring a charge level of each of the first and second redox flow cells;
An information processing unit for calculating a charge difference between the first and second redox flow cells based on the measured charge level; And
And a control unit for controlling electrical connection between the first and second redox flow cells by controlling the DC / AC switching unit based on a difference in charge between the first and second redox flow cells, Flow battery system.
The method of claim 3,
Wherein the control unit controls the pump of each of the first and second redox flow cells to circulate the first electrolyte and the second electrolyte.
The method according to claim 1,
In the idle unit before charging or discharging is performed in the first and second redox flow cells, the DC / AC conversion unit converts the electrical connection between the first and second redox flow cells from DC to AC Redox flow battery system.
Measuring the degree of filling of the first redox flow cell and the second redox flow cell;
Calculating a charge difference between the first and second redox flow cells;
Converting the electrical connections between the first and second redox flow cells from serial to parallel based on the charge difference;
Circulating an electrolyte of each of the first and second redox flow cells for a predetermined period of time to reduce the difference in charge between the first and second redox flow cells;
Converting the electrical connections between the first and second redox flow cells into parallel to serial; And
And charging or discharging the first and second redox flow cells. ≪ Desc / Clms Page number 20 >
The method according to claim 6,
Wherein the step of converting the electrical connections between the first and second redox flow cells from serial to parallel is performed when the charge difference is greater than a first reference value,
Wherein the first reference value is selected within the range of 5% to 20%.
The method according to claim 6,
Wherein the step of converting the electrical connection between the first and second redox flow cells into parallel to serial is performed when the charge difference is less than a second reference value,
Wherein the second reference value is selected from 1% to 10%.
The method according to claim 6,
The step of reducing the charge difference is performed when the first and second redox flow cells are electrically disconnected from an external power source or an external load,
Wherein the charging or discharging step is performed when the first and second redox flow cells are electrically connected to an external power source or an external load.
The method according to claim 6,
Wherein the first and second redox flow cells do not share an electrolyte solution with each other.

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